1. Field of the Invention
The present invention relates to modified polyisocyanate mixtures based on polyisocyanates and polyacrylate units, to a process for preparing them and to their use as a curing component in polyurethane coating compositions.
2. Description of Related Art
With polyurethane coating compositions, particularly if they are to be used in the vehicle, industrial or furniture sectors, especially great value is generally placed on the resistance of such coating compositions to different environmental influences. The criteria are frequently hardness, chemical resistance and solvent resistance, scratch resistance, including what is called “reflow”, light stability and weather resistance.
By “reflow” is meant the ability of a cured coating (film) to compensate for minor film damage (in the μm range), caused by scratching or impact on the film, by cold flow of the coating composition into the damaged site.
To improve the scratch resistance use is frequently made of oligomeric polyisocyanates based on hexamethylene diisocyanate (HDI) as the polyisocyanate component. The polyurethane coating compositions prepared from such components are generally tough and elastic with good reflow. Disadvantages of such coating compositions include the somewhat slow drying at room temperature and slightly elevated temperature, and also the merely moderate acid resistance. Hard, fast-drying polyurethane coating compositions with very good acid resistance are generally obtained with polyisocyanate curatives based on isophorone diisocyanate (IPDI). The scratch resistance and the reflow of such coating compositions, however, are generally inadequate. Moreover, IPDI-based polyisocyanates have a high viscosity and a relatively low isocyanate content.
U.S. Pat. No. 4,419,513 describes isocyanurate polyisocyanates which are obtained by the mixed trimerization of HDI and IPDI. It is disclosed that the mixed trimers have desirable properties in terms of hardness and elasticity. A disadvantageous consequence with these mixed trimers is that, due to the fraction of IPDI, which is necessary for the requisite hardness and rapid physical drying, the amount of isocyanate groups (relative to the molecular weight) is lower than in the case of pure HDI trimers, with attendant economic drawbacks.
EP-A 0 646 608 relates to polyisocyanates which are obtained by the cyclic trimerization of at least one aliphatic or alicyclic diisocyanate either after its reaction with a polyfunctional alcohol or by trimerization in the presence of such an alcohol. Although such polyisocyanates have high functionalities, the fraction of polyfunctional alcohol in the polyisocyanate molecule prepared lowers the weight fraction of isocyanate groups per molecule and, as a consequence of the urethane groups that form, there is a marked increase in viscosity. With regard to the use of the polyisocyanate, this necessitates an economically undesirably high amount of polyisocyanate curative and an increased volume of solvent for adjusting the application viscosity of the coating composition.
U.S. Pat. No. 4,454,317 describes polyisocyanate containing isocyanurate groups which are obtainable by trimerizing HDI. Described by way of example is an HDI trimer having an NCO content of 20.8% by weight and a viscosity of 14 Pas at room temperature. This patent does not disclose anything regarding the possibility of using polyisocyanates of such high viscosity, in combination with suitable polyols, to prepare polyurethane coating compositions having improved chemical resistance.
The modified polyisocyanate mixtures disclosed in DE-A 100 13 187 are notable for a high isocyanate functionality, but this is largely obtained at the expense of the isocyanate content of the respective polyisocyanate. In the preparation of high functionality or high molecular weight polyisocyanates by the oligomerization of diisocyanates by known isocyanate reactions such as biuretization, urethanization, trimerization and allophanatization, large numbers of isocyanate groups are generally consumed for these molecular weight-increasing and functionality-building isocyanate reactions. In general the higher the molecular weight of the polyisocyanate becomes, the more the isocyanate content of the end product falls. This circumstance harbours economic drawbacks.
Therefore, it is an object of the present invention to provide new polyisocyanate compositions, which function as a curing component in polyurethane coating compositions and, in so doing, are able to satisfy the broad spectrum of coating properties that are required, and do not exhibit the stated disadvantages of prior art polyisocyanates. These new polyisocyanate compositions should be variable and should represent an optimum in terms of achievable isocyanate content, molecular weight and functionality.
This object may be achieved with the polyacrylate-modified polyisocyanate of the present invention, which exhibit the required properties. These new polyisocyanates may be obtained by partial reaction of known polyisocyanates with hydroxy-functional unsaturated compounds to form urethane groups and subsequent polymerization of the unsaturated groups and optionally copolymerization with other unsaturated compounds. These new polyisocyanate mixtures are capable of broad variation in terms of their composition, their molecular weight and their functionality and thus in terms of their overall profile of properties.
The modified polyisocyanate mixtures of the invention have very good compatibility with a multitude of polyols and can be formulated to polyurethane coating compositions having a broad spectrum of properties. Particularly advantageous when compared to the corresponding base polyisocyanates have proven to be the markedly improved physical drying and significantly higher solvent resistance and chemical resistance of corresponding polyurethane coating compositions, particularly those based on HDI, without loss of toughness and elasticity, the good reflow or the high scratch resistance.